Wireless communication systems, for example cellular telephony or private mobile radio (PMR) communication systems, typically provide for radio telecommunication links to be arranged between a number of mobile wireless communication units, often referred to as ‘mobile stations’ (MSs). In wireless communication systems, a method of communicating between the MSs typically involves use of one or more intermediary stations to forward a received communication from a first MS to a second MS, as provided in ‘trunked mode’ communication or repeater-based direct mode (DM) communication in a PMR communication system or in a mobile cellular telephone communication system.
In a wireless private mobile radio (PMR) communication system timing mechanisms are typically used. For example, where the wireless PMR communication system operates in accordance with the TErrestrial Trunked RAdio (TETRA) standard defined by the European Telecommunications Standards Institute (ETSI), the TETRA timing structure uses a time division multiple access (TDMA) protocol. Symbol Timing Recovery (STR) is employed and is a procedure by which the receiving MS adjusts its clock timing to match the transmitting MS's timing scheme. Receiving, processing and possibly adapting a clock timing, is performed at least once every multi-frame (eighteen frames) according to the TETRA standard.
It is known for wireless communication units and associated wireless communication systems to utilise Phase Shift Keying (PSK) for the modulation and demodulation of wirelessly transmitted signals. An example of such a communication system is the TETRA system defined by ETSI.
The general principle of most digital modulation schema is that a complex valued baseband signal is selected with desired spectral characteristics. The complex valued baseband signal is then transformed into a real valued high frequency (HF) signal. The spectral characteristics of the HF signal are the same as the spectral characteristics of the baseband signal, for example with respect to bandwidth and shape of the signal. However, in the frequency domain, the signal is shifted into the area around the HF carrier frequency.
In summary, a TETRA baseband signal may be depicted as follows:B(t)=Σakg(t−kT)  [1]where ak represents the kth symbol comprising two bits of information to be transferred, and:ak=ak−1expφj  [2]where φ is equal to a phase shift between two adjacent symbols according to a scheme, as illustrated in Table 1 below:
TABLE 1QPSK phase increment according to recovered bitsBitsPhase increment00Pi/4013*Pi/410−Pi/411−3*Pi/4
wherein:
g(t−kT) is an impulse response of the cosine roll-off filter on the kth symbol, and where a period T is 1/18000 msec, according to the TETRA symbol rate.
FIG. 1 illustrates a graph 100 of an impulse response of a current symbol 105, with a previous symbol located at 110, and a subsequent symbol located at 115. At the times kT the adjacent symbols are not affected by each other as their impulse responses converge to zero (according to the Nyquist criteria).
Each TETRA signal burst contains in the middle thereof a training sequence, which allows a receiver to locate a position of each symbol within the TETRA signal burst, distinguished by strong energy peaks within the base band signal. This is made possible due to a selection of bit patterns in the training sequence, as defined in the TETRA standards.
Upon receipt of a TETRA signal burst, the receiver locates the training sequence in the middle of the TETRA signal burst, and thereby locates the energy peaks of training sequence symbols. The whole TETRA signal burst is then demodulated, and the phase shifts between the neighbouring symbols calculated as follows:
                              ⅇ          φ                =                              A                          k              +              1                                            A            k                                              [        3        ]            where:
Ak+1 and Ak are the (k+1)th and the kth symbols of the received base band signal respectively, and
eφ represents two bits of transmitted information.
A problem for such transmission systems is intra-symbol interference, which in known communication systems is addressed by transmitting symbols at a rate equal to, or slightly above, the Nyquist rate.
Due to an ever-increasing demand for communication bandwidth, it is desirable for the rate at which symbols are transmitted to be increased. However, as is well known in the art, if symbols are transmitted at a rate greater than that of the Nyquist rate, the symbols begin to overlap, causing inter-symbol interference, resulting in a significant increase of the symbol error probability and thereby signal/noise ratio. Such degradation, in the context of reconstructing received signals, is unacceptable. Consequently, the rate at which symbols may be transmitted using known techniques is limited to the Nyquist rate.
Thus, there exists a need for a transmitting communication unit, a receiving communication unit, a communications system and methods of modulating and demodulating phase shift keyed signals that address at least some of the shortcomings of past and present phase shift keying modulation and demodulation techniques and/or mechanisms.